Downhole, or subsurface, pumps are commonly used to lift fluids, such as crude oil, to the surface. The pump is located in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface, to a pump jack device, or beam pump unit. The pump jack is connected to the sucker rod string by way of a polished rod that reciprocates in a stuffing box. The stuffing box provides a seal around the polished rod, allowing the flow of downhole fluids to be directed to an outlet line.
Stuffing boxes for wellhead polished rods typically use friction type seals constructed from a rubber-based material. Common friction type seals used in wellhead stuffing boxes are conical seals, such as shown in U.S. Pat. Nos. 3,084,946 and 4,560,176 and pressure seals. The seals, which are pressurized either by a gland or by well bore pressure, wear and must be periodically replaced.
Replacing conventional seals is fraught with problems; well operators do not like to do so because of the time involved and other reasons. For example, the well must be shut down to stop the movement of the polished rod. Furthermore, before the seals are removed, they must be isolated from the pressure inside of the well. This is accomplished by closing a packer or a blowout preventer located below the stuffing box. Occasionally, the ability of the packer to maintain a seal is compromised by factors such as age or even wear, as the operator may have relied on the packer to compensate for leaking stuffing box seals. If this happens, the packer seal could fail when the operator is removing the stuffing box seals, resulting in a dangerous pressure release of hydrocarbons or gas.
Furthermore, the seals are out of sight in the stuffing box. The operator has no adequate way to determine if the seals are in satisfactory condition, other than to stop the pump jack and open the stuffing box to visually inspect the seals, or to wait for crude oil to leak from the stuffing box. As U.S. Pat. Nos. 5,209,495 and 6,302,401 teach, the leakage of crude oil from the stuffing box is environmentally unsound.
Stuffing boxes can be provided with an injectable packing, such as is taught by Cox, U.S. Pat. No. 4,162,078. Such injectable packing is useful for many pumps, such as the multiplex-type pump, where the reciprocating rod or piston moved along a fixed axis. In a well bore pump however, the axis of movement of the polished rod is itself subject to movement. Any such movement of the polished rod axis causes gaps and shifts in the packing material, resulting in lost seal integrity.